| Summary: | Small modular reactors (SMRs) are gaining interest as a potential solution for cost-effective, carbon-neutral district heat (DH) production. The low pressures and temperatures permit much lighter and cheaper designs than in power plants, and efficiency is high as all heat generated can be sold to customers. In this work, the optimization of the primary heat exchangers in a natural-circulation 50-MW heating reactor concept was carried out to obtain an initial feasibility estimate for the concept for both baseload and load-following operation, as well as to obtain information on the characteristics of an optimized design. Studies on small natural circulation heat-only SMRs and the impact of heat exchanger design on the overall dimensions and economics have not been published before. Although a detailed heat exchanger cost model was used, the results should be considered tentative initial estimates, as much of the cost impact from the heat exchanger design comes from the effect the design has on the pressure vessel dimensions. While more detailed pressure vessel designs and cost functions are needed for final optimization, the feasibility of the concept is shown. Optimization for different load profiles produced near-identical designs, with the downcomer divided approximately in half between the heat exchanger at the top and an empty space at the bottom to maximize the pressure difference available for natural circulation. Although conservative, even pessimistic estimates were used in the absence of detailed cost functions, cost prices of 30–55 EUR/MWh<sub>DH</sub> at a 10% interest rate were obtained, or only 20–40 EUR/MWh<sub>DH</sub> at a 5% interest rate. This indicates potentially good competitiveness for the considered DH SMR concept.
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